I think the overuse of 'sensei' can sometimes lead to the diminishing of the senpai-kohai relationship. I feel like the leadership of senior mudansha, and junior yudansha, is very important to a healthy dojo. Of course, many aikido dojo are relatively small and can't really function with such distinctions...I get that. But I also feel like allowing these senpai to take positions of responsibility for the dojo, themselves, and junior students, is a great opportunity for growth as a persona as well as aikidoka. Some of the biggest influences in my life were my mentors (granted he was yondan at the time) in addition to my sensei.
I guess the correlation to the thread topic I am making is when a shodan or nidan are automatically pushed to a sensei status, that can kind of put a little too big a barrier between other students. There should definitely be a distinction, but I would hope the intimacy of a senpai-kohai relationship can still flourish, regardless of rank.
Of course, mentors exist in the West all the time - and without the sempai-kohai stuff.
Personally, it's been my experience that, in the long run, trying to transplant the sempai-kohai stuff out of Japanese culture causes more problems than any benefits you might get out of it.